What is preventing this code from compiling in Visual C++ 6?

Question

I'm looking at this GitHub repository for a drop-in replacement for std::shared_ptr in Visual C++ 6 to modify some old code but I've found that this library is incompatible due a parser error being thrown by the compiler.

I know that nested template classes and friend classes are supported. But this is a template<class U> which is different from the template of class shared_ptr and also that the friend class is referencing itself.

What is the purpose of this self-referencing templated friend class and how is it being used?

The problem can be demonstrated without compiling the referenced shared_ptr library:

template<class T>
class shared_ptr
{
    template<class U>
    friend class shared_ptr;
};

int main() {
   return 0;
}

The compiler throws the following error:

main.cpp(5) : error C2059: syntax error : '<end Parse>'
        main.cpp(6) : see reference to class template instantiation 'shared_ptr<T>' being compiled
main.cpp(6) : error C2143: syntax error : missing ';' before '}'
        main.cpp(6) : see reference to class template instantiation 'shared_ptr<T>' being compiled
main.cpp(6) : error C2238: unexpected token(s) preceding ';'
        main.cpp(6) : see reference to class template instantiation 'shared_ptr<T>' being compiled

I don't need this library as I've been able to work around the issue by using Boost 1.34.1. But I would like to understand this limitation and what it purpose it serves.